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Both of these particular symbols can be placed over characters using accents. For a full reference, see this Latex wikibook.

In math mode, you would use $\tilde{x}$ for x with a ~ on top, and $\bar{x}$ for x with a bar on top. If you would like the tilde or bar to span multiple characters, you can instead use $\widetilde{xy}$ or $\overline{xy}.

In text mode, you could use \~{x} and \={x} to get these accents over those characters.